Conventionally, hot-melt pressure-sensitive adhesive formulations, generally denoted by HMPSA formulations, for example used in adhesive tape and label applications, have to exhibit a compromise in properties between their use (thermal stability, level of viscosity, and the like) and their physical properties (adhesion, cohesion, resistance to temperature, and the like). Adhesive tapes using HMPSA adhesives have now been produced for more than 50 years.
HMPSA adhesive formulations are usually composed of polymers, of tackifying resins and of oil. It is by adjusting the overall formulation (nature of the ingredients and percentages) that the specifications for a given application are met. In these formulations, the use of the tackifying resin and of the oil makes it possible simultaneously to adjust the properties but also to reduce the overall cost of the formulation, the polymer generally being the most expensive compound.
According to the prior art, see, for example, Ullmann's Encyclopedia of Industrial Chemistry, 5th edition (1995), Vol. A 26, p. 659-660, HMPSAs are preferably based on SIS (styrene-isoprene-styrene) but also SBS (styrene-butadiene-styrene) block copolymers.
In these polymers with the general structure A-B-A, the A blocks are generally thermoplastic in nature with a glass transition temperature (Tg) of greater than 0° C. and preferably than 60° C. and the B blocks are elastomeric in nature with a Tg of less than 0° C., preferably than −30° C.
One of the key parameters to making a success of an HMPSA formulation is to control the compatibility between the tackifying resin and/or the oil, on the one hand, and the block copolymer, on the other hand, in order for these additives to be able to be mixed and thus to selectively modify each of the blocks. In fact, most of the existing products on the market were developed for the purpose of being able to modify either the styrene blocks or the isoprene or butadiene blocks.
Generally, HMPSA formulations comprise between 15% and 40% of polymer, the remainder being composed of tackifying resin and of plasticizer.
As mentioned in U.S. Pat. No. 5,313,041, relating to HMPSAs, SBSs and SISs exhibit the disadvantage of being unstable towards heat and towards ultraviolet (UV) radiation.
Novel ABA block copolymers have thus been developed where the B block is composed of an alkyl acrylate. The advantage of this type of monomer as elastomeric phase relates to better thermal stability and better resistance to UV radiation due to the absence of a double bond and better adhesion results due to the greater polarity.
Such copolymers are disclosed, for example, in Patent EP 0 349 270 B1. The said polymers are prepared by radical polymerization in the presence of an iniferter. It should be noted that all the examples given in this document relate to products comprising a minimum of 75% of phase B.
Given that the solubility parameters of the styrene and alkyl acrylate monomers, in particular the butyl acrylate monomers, are similar, the A-B-A block copolymers for which the A block is essentially based on polystyrene and the B block is based on poly(butyl acrylate) are particularly difficult to formulate. This is because, in order to avoid having miscibilization between the blocks and thus losing the characteristic properties of HMPSAs, it is particularly important to fully control the process for the manufacture of the copolymer so as to have the best phase separation possible. It is also particularly important to properly choose additives, such as the tackifying resins and plasticizers, participating in the formulations for the purpose of selectively modifying the A block or the B block. A hasty selection of the additives can result either in immiscibility with the A and B block or, on the contrary, in high miscibility with the B block and/or the A block which results in a loss in the phase separation and thus in a poor final result in terms of adhesive formulation.
The document JP2001288442 of Nitto Denko Corp. discloses the synthesis of A-B-A copolymers for which the A block is composed of styrene and the B block of an acrylate with a sufficiently high difference in solubility coefficient to have good phase separation during the synthesis. The radical polymerization process employed in this document is a process of ATRP (Atom Transfer Radical Polymerization) type. This technique involves the use of metal derivatives as control agents, which requires the introduction, into the process for the manufacture of the said polymer, of an additional purification stage at the end of the polymerization in order to remove the traces of metal which are present in the reaction medium. It should also be noted that this document is not associated with the formulation of the polymer in order to obtain an adhesive typical of HMPSAs.
The Applicant Company, seeking to solve all the problems described above, has found that adhesive compositions based on block copolymers with a well defined architecture and a well defined chemical structure, such as those described later, and on particularly well suited tackifying resins and on particularly well suited oil exhibit a relatively advantageous combination of the applicative properties and constitute a good solution.